The disclosures of the following priority application is incorporated herein by reference:
Japanese Patent Application No. 11-261420 filed Sep. 16, 1999.
1. Field of the Invention
The present invention relates to a microscope which incorporates an aberration correction optical system.
2. Description of the Related Art
(1) When observing a test piece of transparent or semi-transparent living material (hereinafter termed the subject of observation), this subject of observation is covered over with a cover glass. In the prior art it is known for the objective lens to be designed with considerations of the thickness of the cover glass and moreover a layer of air of predetermined thickness, i.e. a predetermined working distance, to be present between the cover glass and the objective lens. Due to this, spherical aberration occurs if a cover glass is used whose thickness differs from the design value.
Accordingly, in the prior art, an objective lens fitted with a spherical aberration correction lens has been used, and spherical aberration generated due to discrepancy of the thickness of the cover glass has thus been corrected. A correction ring fitted with a graduated scale is provided to this objective lens fitted with a spherical aberration correction lens, and this correction ring is operated according to its scale by an amount just corresponding to the thickness difference of the cover glass.
On the other hand, when observing the interior of a subject of observation, the focus position is adjusted by approaching the objective lens towards the cover glass. In this case, the spherical aberration is eliminated by operating the correction ring according to the amount of shifting of the objective lens. The scale of the correction ring of a prior art objective lens fitted with a spherical aberration correction lens is graduated according to the thickness discrepancy of the cover glass. However, since there may be a discrepancy between the index of refraction of the cover glass and the index of refraction of the subject of observation, it is difficult perfectly to eliminate the spherical aberration with the graduated scale upon the correction ring, and also the operation of correction is complicated.
(2) On the other hand, there is a per se known microscope which constructs a three dimensional image (a vertically sectioned image) by performing tomography upon the above described test piece of transparent or semi-transparent living material (hereinafter termed the subject of observation). With this microscope, a plurality of plan images of the subject of observation are taken while shifting the focus position in the thickness direction of the subject of observation, and a three dimensional image is constructed from these images. When constructing this three dimensional image, a computer performs image processing upon the plurality of plan images, and obtains clear plan images. Deconvolution is one known computer image processing method of this type. This is a method of eliminating image blurring from the plan image which has been obtained, by calculation.
When performing this kind of tomography, the objective lens is shifted in order to vary the focus position inside the subject of observation, and spherical aberration occurs according to the same theory as described above. Since this spherical aberration cannot be eliminated by the deconvolution image processing method described above, the elimination of spherical aberration must be performed with an objective lens fitted with a spherical aberration correction lens. However, each time the objective lens is shifted, i.e. for each focus position, there is the difficulty of operating the correction ring and eliminating the spherical aberration, and it can happen that the spherical aberration is not perfectly eliminated while referring to the graduated scale on the correction ring.
The objective of the present invention is to provide a microscope which can simply corrects the spherical aberration which varies according to the observation position.
The microscope according to the present invention accomplishes the above objective by comprising: an objective lens which focuses upon and observes a specimen mounted upon a stage; an imaging device which receives an optical image of the specimen captured by the objective lens and outputs it as an image signal; a correction optical system provided between the objective lens and the imaging device, which corrects spherical aberration included in the optical image; an optical system drive device which drives the correction optical system to control the amount of spherical aberration correction; and a control section which drives the correction optical system by the optical system drive device so as to vary the amount of spherical aberration correction, based upon the image signal from the imaging device.
The control section may be constituted so as to: image the specimen with the imaging device while varying the amount of spherical aberration correction by the correction optical system; to select an image of which spherical aberration is the least from among a plurality of images which have been imaged by the imaging device; and to drive the correction optical system so as to bring the spherical aberration correction amount to the value when the selected image was imaged.
Furthermore, the control section may select, from among the plurality of images which have been imaged, an image of which the contrast is the greatest as the image of which spherical aberration is the least.
In the above described microscope, it is preferable to be further included an electrically operated raising and lowering device which raises and lowers the stage. In this case, the control section sets the stage to a plurality of observation positions by raising and lowering it by the electrically operated raising and lowering device; for each observation position, images the specimen with the imaging device while varying the amount of spherical aberration correction by the correction optical system with the optical system drive device; and stores in an image storage device an image of which spherical aberration is the least from among the plurality of images which have been imaged by the imaging device.
With a microscope which includes a focus detection device which calculates the focusing state of the objective lens based upon a plurality of the image signals imaged while shifting the stage along the optical axis, it is possible to shift the stage by driving an electrically operated raising and lowering device based upon the signal detected by the focus detection device. The focus detection device is able to calculate the stage position corresponding to an image of which the contrast is the greatest among the plurality of images which have been imaged based upon the image signals as the focused state.
With this type of microscope, it is preferable for the control section to select, from among the plurality of images which have been imaged, an image of which the contrast is the greatest as the image of which spherical aberration is the least.
It is possible for the control section: to image a test piece having an optical characteristics equivalent to the specimen with the imaging device; to select an image of which spherical aberration is the least from among a plurality of images which have been imaged while varying the amount of spherical aberration correction; to store the aberration correction position of the correction optical system when the selected image was obtained; and, when the specimen is being imaged by the imaging device, to read out the aberration correction position which has been stored and to drive the correction optical system. The test piece may have an equivalent index of refraction to the specimen. And the control section may be constituted so as to detect the temperature of the test piece when storing the aberration correction position using the test piece, and correct the aberration correction position based upon this detected temperature when storing it. In this case, it may calculate the difference between the temperature determined when storing the aberration correction position using the test piece, and the temperature when imaging the specimen; and may correct the aberration correction position according to the temperature difference.
The microscope according to the present invention may alternatively accomplish the above objective by comprising: an objective lens which focuses upon and observes a specimen mounted upon a stage; an imaging device which receives an optical image of the specimen captured by the objective lens and outputs it as an image signal; a correction optical system provided between the objective lens and the imaging device, which corrects spherical aberration included in the optical image; an optical system drive device which drives the correction optical system to control the amount of spherical aberration correction; and a control section which performs imaging with the spherical aberration correction amount when an image was obtained of which spherical aberration was the least among the plurality of images which were imaged by the imaging device while varying the spherical aberration correction amount.